1. Field of the Invention
The present invention relates to a device at an optical communications network which supplies a virtual attenuation-free optical connection by utilizing a driver laser which transmits an optical pump signal which activates amplifying elements which are placed along a signal line. The amplifying elements give an essentially noiseless amplification by utilizing a doped fibre, and by that a compensation of the in the signal line intrinsic attenuation is obtained in a way that a virtually attenuation-free optical connection is obtained.
2. Discussion of the Background
A problem at transmission of optical signals in an optical communications network is that often too much noise is obtained from the big and besides sparsely located signal amplifiers. This results in a restricted transmission capacity at the transmission of optical signals and also requires a complex and expensive terminal equipment both on the amplifier side and the receiver side.
A technology which is used today is to place fibre optical amplifiers with high amplification in special pump stations at certain points along the signal fibre. The pump stations contains the pump laser and an active fibre in the same place. The amplifiers are placed comparatively sparsely, usually at a distance of 50-100 km. The high amplification results in that the amplifiers produce much noise and that they must be specially equipped with isolators, filters etc to avoid disturbances like self-oscillations and saturation. It is too expensive to place these amplifiers close, because of installation costs and supervision and maintenance. U.S. Pat. No. 5,115,338 describes a system of this type. Also U.S. Pat. No. 4,699,452 describes a similar system which utilizes the Raman principle instead of doped fibre as amplifying element.
The big noise contribution from the amplifiers results in that the optical transmitter laser must produce high output power, often via a special transmitter booster. High power levels results in that the fibre becomes unlinear as signal carrier. Such an unlinear transmission channel leads to strong restrictions of the range, how high data rates that can be used, and the number of optical channels which can be transmitted in the signal fibre at the same time.
Another method is to distribute the amplification along the signal fibre by applying doped fibres at points along the signal fibre and in this way reduce the noise contributions. At this the power of the pump laser is driven directly into one end of the signal fibre. The high pump power which is required, results in further problems with unlinear transmission, as in the case above. An example of this method is described in U.S. Pat. No. 5,039,199.
A similar method is to make the whole signal fibre as a low-doped fibre. This method is however impaired by the same problem. An example of this method is given in U.S. Pat. No. 5,058,974 and a combination of these methods is given in EP 0 408 394.
EP 0 387 075 shows that a doped fibre can be used in optical transmitters as an alternative to the semiconductor laser. The document shows a way to more efficiently utilize the pump power necessary to drive the doped fibre.
U.S. Pat. No. 4,546,476 describes a fibre optical amplifier where the coupler and the active fibre are arranged as one component.